Optical pickup compatible with a plurality of types of optical disks having different thicknesses

ABSTRACT

In an optical pickup device ( 10 ) including a laser light source ( 1 ) and an objective lens ( 7 ), an optical device ( 6 ) is provided that transmits a laser beam of 635 nm in wavelength emitted from a first semiconductor laser ( 1 A) straightforwardly into the objective lens ( 7 ) while maintaining its incident intensity, and that selectively diffracts a laser beam of 780 nm in wavelength emitted from a second semiconductor laser ( 1 B) to a desired direction while maintaining its incident intensity and directs only the predetermined center portion of the laser beam into the objective lens ( 7 ). The optical pickup device ( 10 ) records or reproduces a signal onto or from a plurality of types of optical disks differing in substrate thickness while directing a laser beam of sufficient intensity onto a signal recording plane.

TECHNICAL FIELD

The present invention relates to optical pickup devices. Moreparticularly, the present invention relates to an optical pickup devicethat directs a laser beam of sufficient intensity for signal recordingonto an optical recording medium to record and/or reproduce a signalonto/from a plurality of types of optical recording media.

BACKGROUND ART

Optical disks of approximately 1.2 mm in thickness to read outinformation using a semiconductor laser such as a CD-ROM (CompactDisk-Read Only Memory) are proposed. By carrying out focus servo andtracking servo on the objective lens for pickup with respect to thistype of optical disk, a laser beam is directed to a pit train on thesignal recording plane from which a signal is reproduced.

A CD-R (Compact Disk-Recordable) is available that has a recordingdensity identical to that of the CD and that allows recording only once.A laser beam of 780 nm in wavelength is employed in recording andreproducting signals thereof.

Recently, the density is further increased to record a motion picturefor a long period of time. For example, a DVD (Digital Video Disk) thatrecords 4.7 Gbytes of information on one plane of an optical disk havinga diameter of 12 cm that is identical to that of the CD-ROM iscommercially available. The thickness of a DVD is approximately 0.6 mm.By fixing these planes together, 9.4 Gbytes of information can berecorded in one disk.

Attention is focused on a magneto-optical recording medium as arewritable recording medium of great storage capacity and highreliability. The magneto-optical recording media are now applied ascomputer memories and the like. Standardization of a magneto-opticalrecording medium having a storage capacity of 6.0 Gbytes (AS-MO(Advanced Storage Magneto Optical Disk) standard) is in progress to beprovided for actual usage. This magneto-optical recording medium has thesignal reproduced by the MSR (Magnetically Induced Super Resolution)method. More specifically, a laser beam is projected to transfer themagnetic domain of the recording layer of the magneto-optical recordingmedium to a reproduction layer and also forming a detection window inthe reproduction layer to allow detection of only the transferredmagnetic domain. The transferred magnetic domain is detected from theformed detection window. A laser beam of 600-700 nm in wavelength isemployed for recording and/or reproducing a signal onto and/or from themagneto-optical recording medium.

It is expected that there will be the coexistence of CDs, CD-Rs, DVDsand magneto-optical recording media in the future. The need arises foran optical pickup device that can reproduce information from suchoptical disks and that can record a signal onto a recordable opticaldisk. WO 98/19303 discloses an optical pickup device that allowscompatible reproduction between a CD-R and a DVD.

The proposed CD-R/DVD compatible pickup includes a semiconductor lasergenerating a laser beam of 635 nm in wavelength for reproduction of aDVD and a semiconductor laser generating a laser beam of 780 nm inwavelength for recording and reproduction of a CD-R. When a signal is tobe recorded onto or reproduced from a CD-R using a laser beam of 780 nmin wavelength, the laser beam is diffracted and a desired diffractedlight thereof, for example only the first order light, is introducedinto the objective lens to collect light in order to correct aberrationcaused by difference in the thickness of the substrate.

Therefore, the zero order light or minus first order light could not beused effectively. There was a problem that a laser beam sufficient inintensity for recording could not be obtained at the signal recordingplane of the CD-R.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an optical pickupdevice capable of recording and/or reproducing a signal onto/from aplurality of types of optical disks of different thickness, suppressinglaser beam loss to the minimum.

According to an aspect of the present invention, an optical pickupdevice recording and/or reproducing a signal onto/from a first opticaldisk and a second optical disk thicker than the first optical diskincludes a light source, an objective lens, and an optical device. Thelight source generates a laser beam. The objective lens is locatedopposite to the first and second optical disks. The optical device isarranged between the light source and the objective lens to transmit thelaser beam from the light source straightforwardly during recording orreproduction of the first optical disk, and bending substantially theentire laser beam from the light source and increasing the diameterthereof to guide the center portion of the laser beam towards theobjective lens and the peripheral portion of the laser beam outside theobjective lens during recording or reproduction of the second disk.

In the optical pickup device, the optical device bends substantially theentire laser beam from the light source so that only the center portionof the laser beam is guided to the objective lens during recording orreproduction of the second optical disk, so that most of the laser beamcan be used effectively with the exception that the peripheral portionis lost. A signal can be recorded onto the first and second opticaldisks or a signal can be reproduced from the first and second opticaldisks while suppressing loss of the laser beam at the minimum.

Preferably, the optical device includes a first optical member and asecond optical member. The first optical member has a first refractiveindex. The second optical member is in contact with the first opticalmember, and has the first refractive index during recording orreproduction of the first optical disk, and has a second refractiveindex differing from the first refractive index during recording orreproduction of the second optical disk. During recording orreproduction of the first optical disk, the entire optical device hasthe first refractive index. Therefore, the laser beam from the lightsource is transmitted straightforwardly. In contrast, the first andsecond optical members have different refraction indexes duringrecording or reproduction of the second optical disk. Therefore, theoptical device diffracts or refracts the laser beam from the lightsource.

Further preferably, the light source generates a first laser beam havinga first wavelength during recording or reproduction of the first opticaldisk, and generates a second laser beam having a second wavelengthdiffering from the first wavelength during recording or reproduction ofthe second optical disk. The first optical member has a first refractiveindex for the first and second wavelengths. The second optical memberhas the first refractive index for the first wavelength and the secondrefractive index for the second wavelength. Since the refractive indexof the second optical member changes according to the wavelength, thelaser beam can be transmitted straightforwardly or bent withoutmechanical switching.

Also preferably, the first optical member includes a hologram formed tocome into contact with the second optical member. Therefore, the opticaldevice diffracts the laser beam by interference during recording orreproduction of the second optical disk.

Further preferably, the first optical member is arranged at the lightsource side. The second optical member is arranged at the objective lensside. The first refractive index is higher than the second refractiveindex. The hologram includes a plurality of annular projections formedconcentrically. The pitch of the annular projections become smaller astowards the outer circumference. Therefore, the optical device diffractsthe laser beam at a greater angle as towards the circumference.

Preferably, each of the annular projections has a triangular crosssection radially. Therefore, the optical device can diffract theincident laser beam in a desired direction without generating 0 order or−1 order diffracted light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of an optical pickup device of the presentinvention.

FIG. 2 is a prospective view of the optical device of FIG. 1.

FIGS. 3A and 3B are a sectional view and a plan view, respectively, ofthe optical device of FIG. 2.

FIGS. 4A and 4B are diagrams to describe the property of the opticaldevice of FIG. 1 when the wavelength is 635 nm and 780 nm, respectively,and FIG. 4C is a diagram to describe the property of the optical devicehaving a hologram of a stepped shape in cross section.

FIG. 5 is a diagram to describe the path of a laser beam of 780 nm inwavelength from the optical device to the objective lens.

FIG. 6 is a diagram to describe the recording or reproduction operationof an optical disk having a substrate thickness of 0.6 mm using theoptical pickup device of FIG. 1.

FIG. 7 is a diagram to describe the recording or reproduction operationof an optical disk having a substrate thickness of 1.2 mm using theoptical pickup device of FIG. 1.

FIG. 8 is a prospective view of another optical device of the presentinvention.

FIG. 9 is a cross sectional view of the optical device of FIG. 8.

FIG. 10 shows a structure of another optical pickup device of thepresent invention.

FIG. 11 is a cross sectional view of the optical device of FIG. 10.

FIG. 12 is a diagram to describe the property of the optical device ofFIG. 10 when voltage is applied.

FIG. 13 is a diagram to describe the property of the optical device ofFIG. 10 when voltage is not applied.

FIG. 14 is a diagram to describe the recording or reproduction operationof an optical disk having a substrate thickness of 0.6 mm using theoptical pickup device of FIG. 10.

FIG. 15 is a diagram to describe the reproduction operation of anoptical disk having a substrate thickness of 1.2 mm using the opticalpickup device of FIG. 10.

FIG. 16 shows a structure of a further another optical pickup device ofthe present invention.

FIG. 17 is a diagram to describe the rising mirror of FIG. 16.

FIG. 18 is a perspective view of a laser beam emitted from a laser lightsource, rendered parallel by the collimator lens, and entering theobjective lens.

FIG. 19 shows the intensity distribution of the laser beam of FIG. 18.

FIG. 20 shows the intensity distribution of the laser beam along crosssection X-X′ of FIG. 18.

FIG. 21 shows the intensity distribution of the laser beam along crosssection Y-Y′ of FIG. 18.

FIG. 22 is a diagram to describe the rim strength when the priority isgiven on the efficiency.

FIG. 23 shows the intensity distribution of the laser beam along crosssection X—X of FIG. 22.

FIG. 24 shows the intensity distribution of the laser beam along crosssection Y-Y′ of FIG. 22.

FIG. 25 is a diagram to describe the rim intensity when priority isgiven on the spot size.

FIG. 26 shows the intensity distribution of the laser beam along crosssection X-X′ of FIG. 25.

FIG. 27 shows the intensity distribution of the laser beam along crosssection Y-Y′ of FIG. 25.

FIG. 28 is a diagram to describe the rim intensity suitable for anoptical pickup device of eightfold speed.

FIG. 29 shows the intensity distribution of a laser beam along crosssection X-X′ of FIG. 28.

FIG. 30 shows the intensity distribution of a laser beam along crosssection Y—Y of FIG. 28.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described with reference tothe drawings. In the drawings, the same or corresponding components havethe same reference characters allotted, and their description will notbe repeated.

Referring to FIG. 1, an optical pickup device 10 of the presentinvention includes a laser light source 1, a collimator lens 2, apolarization beam splitter 3, a half-wave plate 4, a rising mirror 5, anoptical device 6, an objective lens 7, a collective lens 8, and aphotodetector 9.

Laser light source 1 includes a first semiconductor laser 1A generatinga laser beam of 635 nm in wavelength (tolerance±15 nm, the same applieshereinafter), and a second semiconductor laser 1B generating a laserbeam of 780 nm in wavelength (tolerance±15 nm, the same applieshereinafter). A laser drive circuit not shown selectively drives firstsemiconductor laser 1A and second semiconductor laser 1B to selectivelygenerate a laser beam of 635 nm in wavelength and a laser beam of 780 nmin wavelength.

Collimator lens 2 renders the laser beam from laser light source 1parallel. Polarization beam splitter 3 transmits the laser beam fromcollimator lens 2, and reflects the reflected light from an optical disk11 (or 110) towards photodetector 9. Half-wave plate 4 rotates the planeof polarization of the laser beam 90 degrees and transmits the laserbeam. Rising mirror 5 reflects the laser beam passing through half-waveplate 4 towards optical disk 11 (or 110).

Optical device 6 transmits the laser beam of 635 nm in wavelengthstraightforwardly into objective lens 7 while maintaining the incidentintensity, and diffracts the laser beam of 780 nm in wavelength towardsa desired direction to increase the diameter and enter the centerportion into objective lens 7 and bend the peripheral portion outsideobjective lens 7.

Objective lens 7 is located opposite to optical disk 11 (or 110) tofocus the laser beam from optical device 6 to direct the laser beam ontoa signal recording plane 11 a (or 110 a) of optical disk 11 (or 110).Objective lens 7 is designed corresponding to an optical disk 11 havinga substrate thickness of 0.6 mm. The numerical aperture is 0.6(tolerance ±0.05). Collective lens 8 collects the laser beam reflectedat polarization beam splitter 3. Photo detector 9 detects the laser beamcollected by collective lens 8.

Optical pickup device 10 reproduces a signal from a DVD 11 having asubstrate thickness of 0.6 mm, and records/reproduces a signal to/from aCD-R 110 having a substrate thickness of 1.2 mm. When a signal is to bereproduced from DVD 11, a laser beam having a wavelength of 635 nm isoutput from laser light source 1. When a signal is to berecorded/reproduced with respect to CD-R, a laser beam having awavelength of 780 nm is generated from laser light source 1.

Particularly in the case where a signal is to be recorded on CD-R 110 aswill be described afterwards, optical pickup device 10 has a beamfocused on a signal recording plane 110 a of CD-R 110 for recording withlittle degradation in the power of the laser beam of 780 nm inwavelength emitted from second semiconductor laser 1B.

Details of optical device 6 will be described hereinafter with referenceto FIGS. 2, 3A and 3B.

Referring to FIG. 2, optical device 6 includes a first optical member 60formed of a transmittive substrate such as of glass, and a secondoptical member 61 formed to cover first optical member 60. A pluralityof annular projections 602 are formed at a predetermined distanceconcentrically about an optical axis L0 on the main surface of firstoptical member 60. Projections 602 form a hologram 601. Projection 602is formed of, for example, TiO₂, and has the same refractive index of2.3 with respect to the laser beam of 635 nm in wavelength and the laserbeam of 780 nm in wavelength. Second optical member 61 is formed of, forexample, silicon nitride (SiN), silicon carbide (SiC), and has therefractive index of 2.3 for the laser beam of 635 nm in wavelength andthe refractive index of 1.8 for the laser beam of 780 nm in wavelength.

The cross sectional structure of optical device 6 at an arbitrary planeincluding optical axis L0 will be described with reference to FIG. 3A.Projections 602 having the shape of a right triangle are formedsymmetrically with respect to optical axis L0 at a predeterminedinterval at the surface of first optical member 60. Projections 602 havethe height of 0.337 μm, the interval of 296.43 μm at the innermostcircumferential region and 31.256 μm at the outermost circumferentialregion. The pitch becomes gradually narrower from the innercircumference towards the outer circumference. Second optical member 61is formed to come into contact with the main surface of first opticalmember 60 forming hologram 601. First optical member 60 is arranged atthe light source 1 side whereas second optical member 61 is arranged atthe objective lens 7 side.

FIG. 3B is a plan view of the structure of optical device 6. Hologram601 is formed of a plurality annular projections 602 concentrically atthe surface of transmittive substrate 60. It is apparent that the pitchof annular projections 602 becomes smaller as towards the outercircumference. The hologram lens provided by Aerial Imaging Corporation(U.S.A.) can be employed as first optical member 60.

The optical property of optical device 6 will be described withreference to FIGS. 4A and 4B. Referring to FIG. 4A, projection 602 andsecond optical member 61 both have a refractive index of 2.3 withrespect to the laser beam of 635 nm in wavelength. Therefore, laser beamLB1 of 635 nm in wavelength that enters optical device 6 is directlytransmitted as laser beam LB1 without being diffracted at optical device6. As a result, the laser beam of 635 nm in wavelength will not bereduced in power even if transmitted through optical device 6.

Referring to FIG. 4B, projection 602 has a refractive index of 2.3 andsecond optical member 61 has a refractive index of 1.8 with respect tothe laser beam of 780 nm in wavelength. It is to be noted thatprojection 602 has a gentle slope 603 at the interface with secondoptical member 61. Therefore, laser beam LB2 of 780 nm in wavelengththat enters optical device 6 is diffracted towards the outer side fromthe optical axis via slope 603 when entering second optical member 61from first optical member 60 to be transmitted as diffracted light LB3from optical device 6.

FIG. 4C corresponds to the case where first optical member 60 hasstep-graded projections 40 at the interface with second optical member61. Laser beam LB2 of 780 nm in wavelength is diffracted into 0 orderlight LB20, +first order light LB21 and −1 order light LB22 whenentering second optical member 61 from first optical member 60.Therefore, the power of the laser beam will be reduced when only one ofthe three diffracted light LB20, LB21 and LB22 is employed. However,gentle slope 603 of projection 602 causes laser beam LB2 of 780 nm inwavelength to be diffracted almost 100% to first order light LB3 byoptical device 6. Therefore, there is little reduction in the power oflaser beam LB2 passing through optical device 6.

Referring to FIG. 5, laser beam LB2 of 780 nm in wavelength incident tooptical device 6 is diffracted in a desired direction to be transmittedas diffracted light LB3 from optical device 6. As to diffracted lightLB3 passing through optical device 6, peripheral portion LB3EX does notenter objective lens 7. Only a predetermined center portion LB3IN entersprojection lens 7. Therefore, optical device 6 works on laser beam LB2of 780 nm in wavelength to diffract the laser beam to diffracted lightLB3 of a desired direction, and directs only predetermined centerportion LB3IN into objective lens 7. Since the distance betweenrespective annular projections 602 of optical device 6 becomes graduallysmaller from the center towards the periphery, the angle of diffractiondiffers between the center portion and the peripheral portion. Opticaldevice 6 functions in a manner of diffracting laser beam using a lens.

The efficiency η_(m) of m order diffracted light by the hologram isgenerally represented by the following equation (1). $\begin{matrix}{\eta_{m} = {{\frac{1}{T}{\int_{0}^{T}{{A(x)}\exp \quad \left\{ {\quad \psi \quad (x)} \right\} \exp \quad \left( {{- }\quad \frac{2\quad \pi \quad {mx}}{T}}\quad \right){x}}}}}^{2}} & (1)\end{matrix}$

where T is the hologram cycle (here, the pitch of annular projections602), A(x) is the transmittance, x is the position on the hologram, ψ(x)is the phase difference function, and m is the order.

Since hologram 601 is the kinoform type formed of a plurality of annularprojections 602 having a triangular cross section, the phase differencefunction ψ(x) is represented by the following equation (2).$\begin{matrix}{{{\psi \quad (x)} = {\frac{d\quad \Delta \quad n}{T} \times \frac{2\quad \pi}{\lambda} \times x}},\quad {m = 1},\quad {{A(x)} = 1}} & (2)\end{matrix}$

In equation (2), d is the height of projection 602, Δn is the differencein the refractive index, and λ is the wavelength.

When A(x)=1, the diffraction efficiency of the first order light isrepresented by the following equation (3). $\begin{matrix}\begin{matrix}{\eta_{i} = \quad {{\frac{1}{T}{\int_{0}^{T}{{\exp \left( {\quad \frac{d\quad \Delta \quad n}{T}\quad \frac{2\quad \pi}{\lambda}x} \right)}\exp \quad \left( {{- }\quad \frac{2\quad \pi \quad x}{T}} \right)\quad {x}}}}}^{2}} \\{= \quad {{{\frac{1}{T}{\int_{0}^{T}{\exp \left\{ {\quad \frac{2\quad \pi \quad {x\left( {\frac{d\quad \Delta \quad n}{\lambda} - 1} \right)}}{T}} \right\} {x}}}}}^{2}\quad \left( {{\frac{d\quad \Delta \quad n}{\lambda} - 1} = A} \right)}} \\{= \quad {{\frac{1}{T}{\int_{0}^{T}{\left( {{\cos \quad \quad \frac{2\quad \pi \quad {xA}}{T}} + {\quad \sin \quad \quad \frac{2\quad \pi \quad {xA}}{T}}} \right){x}}}}}^{2}} \\{= \quad {{\frac{1}{T} \times \frac{T}{2\quad \pi \quad A}\left\{ {\left\lbrack {\sin \quad \frac{2\quad \pi \quad {xA}}{T}} \right\rbrack_{0}^{T} + {\left\lbrack {{- \cos}\quad \frac{2\quad \pi \quad {xA}}{T}} \right\rbrack}_{0}^{T}} \right\}}}^{2}} \\{= \quad {{\frac{1}{2\quad \pi \quad A}\left\{ {\left( {{\sin \quad 2\quad \pi \quad A} - 0} \right) + {\left( {{{- \cos}\quad 2\quad \pi \quad A} + 1} \right)}} \right\}}}^{2}} \\{= \quad {\frac{1}{\left( {2\quad \pi \quad A} \right)^{2}}\left( {{\sin^{2}2\quad \pi \quad A} + {\cos^{2}2\quad \pi \quad A} - {2\quad \cos \quad 2\quad \pi \quad A} + 1} \right)}} \\{= \quad \frac{2 - {2\quad \cos \quad 2\quad \pi \quad A}}{\left( {2\quad \pi \quad A} \right)^{2}}} \\{= \quad \frac{4\quad \sin^{2}\pi \quad A}{\left( {2\quad \pi \quad A} \right)^{2}}} \\{= \quad {\frac{\sin^{2}\pi \quad A}{\left( {\pi \quad A} \right)^{2}}\quad \left( {{\sin \quad {c(x)}} = \frac{\sin \quad \left( {\pi \quad x} \right)}{\pi \quad x}} \right)}} \\{= \quad {\sin \quad c^{2}A}} \\{= \quad {\sin \quad {c^{2}\left( {\frac{d\quad \Delta \quad n}{\lambda} - 1} \right)}}} \\{= \quad {\sin \quad {c^{2}\left( \frac{{d\quad \Delta \quad n} - \lambda}{\lambda} \right)}}}\end{matrix} & (3)\end{matrix}$

It is apparent from the above that optical device 6 converts laser beamLB2 of 780 nm in wavelength into first order diffracted light LB3 athigh efficiency. Since the pitch of annular projections 602 becomessmaller towards the outer circumference, the laser beam passing throughthe center of optical device 6 travels straightforwardly while the laserbeam at the periphery is bent at a greater angle as towards the outercircumference. Therefore, the center portion of laser beam LB2 entersobjective lens 7 whereas the peripheral portion of laser beam LB2 isdeviated from objective lens 7. Thus, optical device 6 can diffractsubstantially the entire laser beam LB2 of 780 nm in a desired directionto enter objective lens 7 except for the peripheral portion thereof.

The operation of reproducing a signal from DVD 11 having a substratethickness of 0.6 mm using optical pickup device 10 will be describedwith reference to FIG. 6. In the case of reproducing a signal from DVD11, first semiconductor laser 1A of laser light source 1 is selectivelydriven. The laser beam of 635 nm in wavelength emitted from laser lightsource 1 is rendered parallel by collimator lens 2, and passes throughpolarization beam splitter 3. The light from polarization beam splitter3 has its plane of polarization rotated 90 degrees by half-wave plate 4to enter rising mirror 5. Here, the laser beam passes throughpolarization beam splitter 3 and half-wave plate 4 at the transmittanceof approximately 98%. Therefore, there is little reduction in power bythe passage through polarization beam splitter 3 and half-wave plate 4.

The laser beam incident on rising mirror 5 is reflected almost 100% andenters optical device 6. The laser beam passes through optical device 6while maintaining its incident intensity and then enters objective lens7. The laser beam is focused by objective lens 7 to be projected onsignal recording plane 11 a of DVD 11. The light reflected from signalrecording plane 11 a returns to half-wave plate 4 through objective lens7, optical device 6 and rising mirror 5. The light has its plane ofpolarization rotates 90 degrees at half-wave plate 4, and then enterspolarization beam splitter 3. The reflected light incident onpolarization beam splitter 3 is reflected almost 100% at polarizationbeam splitter 3 to enter collective lens 8 since the plane ofpolarization is rotated 180 degrees than the case where the light enterspolarization beam splitter 3 from collimator lens 2. The light incollective lens 8 is collected and directed to photodetector 9 fordetection.

By using optical pickup device 10, a signal can be recorded and/orreproduced onto/from a magneto-optical recording medium that is arecordable optical disk. In this case, a signal can be recorded with theintensity of light equal to the level of the light immediately outputfrom first semiconductor laser 1A.

Referring to FIG. 7, the operation of recording and/or reproducing asignal onto and/or from a CD-R having a substrate thickness of 1.2 mmusing optical pickup device 10 will be described hereinafter. In therecording and/or reproduction operation with respect to a CD-R, secondsemiconductor laser 1B of laser light source 1 is selectively driven.

First, the signal recording operation will be described. When a signalis to be recorded on a CD-R 110, a strong laser beam of 70 mW is outputfrom second semiconductor laser 1B. The laser beam of 780 nm inwavelength output from laser light source 1 is rendered parallel bycollimator lens 2 and passes through polarization beam splitter 3. Athalf-wave plate 4, the plane of polarization of the light is rotated 90degrees and then enters rising mirror 5. The laser beam passes throughpolarization beam splitter 3 and half-wave plate 4 at the transmittanceof approximately 98%. Therefore, there is little reduction in the powerduring passage of polarization beam splitter 3 and half-wave plate 4.

The laser beam entering rising mirror 5 is reflected almost 100% andenters optical device 6. The laser beam in optical device 6 isdiffracted while maintaining its incident intensity. Only thepredetermined inner portion of the laser beam enters objective lens 7.The laser beam in objective lens 7 is focused and projected onto signalrecording plane 110 a of CD-R 110. It is to be noted that the laser beamis modulated by the record signal. Therefore, a modulated laser beam of780 nm in wavelength is projected onto signal recording plane 110 a,whereby a signal is recorded.

The laser beam of 780 nm in wavelength emitted from second semiconductorlaser 1B at the power of 70 mW has its intensity reduced byapproximately 2% by passage through polarization beam splitter 3 andhalf-wave plate 4, and enters optical device 6. The laser beam isdiffracted and transmitted through optical device 6 while maintainingits intensity. Only the predetermined center portion of light entersobjective lens 7. The predetermined center portion is the region wherethe effective numerical aperture of objective lens 7 of the numericalaperture of 0.6 is in the range of 0.50-0.53. When the effective lightflux of the laser beam of 780 nm in wavelength is 4.46 mm, the diameterof the predetermined center portion where the effective numericalaperture of objective lens 7 is in the range of 0.50-0.53 is 3.2-3.4 mm.Therefore, the intensity of the laser beam incident on objective lens 7is 70 mW ×0.98 ×(center portion diameter effectively used/effectivelight flux of laser beam) =49˜52 mW. Therefore, by using optical pickupdevice 10, a laser beam of 780 nm in wavelength can be projected onsignal recording plane 110 a of CD-R 110 with little reduction in theintensity level to that right after emission from second semiconductorlaser 1B. Therefore, signal recording can be carried out correctly.

Next, the operation of signal reproduction will be described. When asignal is to be reproduced from CD-R 110, a laser beam of 12 mW isemitted from second semiconductor laser 1B. The laser beam of 780 nmemitted from laser source 1 is directed to signal recording plane 110 ofCD-R 110 with little reduction in the intensity, as described above. Thereflected light from signal recording plane 110 a is guided tophotodetector 9 as described with reference to FIG. 6, and a signal isreproduced.

Another optical device 80 employed in optical pickup device 10 will bedescribed with reference to FIG. 8. Optical device 80 includes atransparent first optical member 810 and a transparent second opticalmember 801. First optical member 810 is arranged at the objective lens 7side, and has a concave curve plane 802 in contact with second opticalmember 801. Second optical member 801 is arranged at the light source 1side, and has a convex curve plane 802 in contact with first opticalmember 810. Second optical member 802 has a refractive index of 2.3 fora laser beam of 635 nm in wavelength and a refractive index of 1.8 for alaser beam of 780 nm in wavelength. First optical member 810 has thesame refractive index of 2.3 for a laser beam of 635 nm in wavelengthand a laser beam of 780 nm in wavelength. Second optical member 801 isformed of, for example, SiN. First optical member is formed of, forexample, TiO₂.

The cross sectional shape of optical device 80 at an arbitrary planeincluding optical axis L0 will be described with reference to FIG. 9.The interface 802 between first optical member 810 and second opticalmember 801 has a dome-like aspheric surface protruding towards firstoptical member 810. Although interface 802 maybe a spheric surface, itis desirable to correct the spheric surface slightly to reduceaberration. Since first and second optical members 810 and 801 have thesame refractive index 2.3 with respect to a laser beam of 635 nm inwavelength, the laser beam of 635 nm will not be diffracted by opticaldevice 80 and is transmitted directly through optical device 80. Incontrast, the second optical member 801 has a refractive index of 1.8and the first optical member 810 has a refractive index of 2.3 withrespect to a laser beam of 780 nm in wavelength. Since the interface 802between first and second optical members 810 and 801 has a dome-likeaspheric surface as described above, laser beam LB2 of 780 nm inwavelength is diffracted outwards from the optical axis at opticaldevice 80 and output therefrom as diffracted light LB4. In this case,the intensity of laser beam LB2 is substantially equal to that ofdiffracted light LB4.

Reproduction of a DVD and recording and/or reproduction of a CD-R can becarried out as described above even in the case where optical device 80is used instead of optical device 6 of optical pickup device 10.

Specific examples of first optical members 601 and 810 and secondoptical members 61 and 801 forming optical devices 6 and 80 are notlimited to those described above. In the case of optical device 6, firstand second optical members 60 and 61 have the same first refractiveindex n1 with respect to a laser beam of 635 nm in wavelength, and firstoptical member 60 and second optical member 61 have a first refractiveindex n1 and a second refractive index n2 smaller than the firstrefractive index n1, respectively, with respect to a laser beam of 780nm in wavelength.

When in the case of optical device 80, first and second optical members810 and 801 have the same first refractive index n1 for a laser beam of635 nm in wavelength, and first and second optical members 810 and 801have the first refractive index n1 and the second refractive index n2smaller than the first refractive index n1, respectively, with respectto the laser beam of 780 nm.

Although the substrate portion of the first optical member 60 andprojection 602 are formed separately in optical device 6, they may beformed integrally of the same material. Similarly, optical device 80 mayhave the substrate portion of second optical member 801 and thedome-like projection of second apparatus member 801 formed integrally ofthe same material.

In other words, optical devices 6 and 80 are arbitrary as long as alaser beam can be diffracted selectively in a desired direction whilemaintaining incident intensity corresponding to the wavelength of thelaser beam.

Also, the semiconductor laser mounted in optical pickup device 10 is notlimited to that emitting a laser beam of 635 nm in wavelength and alaser beam of 780 nm in wavelength. A semiconductor laser that emitslaser beams of two other different wavelengths can be employed.

Furthermore, the optical pickup device of the present invention is notlimited to that emitting laser beams of two different wavelengths. Onethat emits a laser beam of one wavelength can be used.

Referring to FIG. 10, a structure of another optical pickup device 20according to the present invention will be described. Optical pickupdevice 20 has a structure similar to that of optical pickup device 10,provided that a laser beam source 100 and an optical device 200 areemployed instead of laser light source 1 and optical device 6,respectively. Laser light source 100 generates only a laser beam of 635nm in wavelength.

The structure of optical device 200 will be described with reference tothe cross sectional view of FIG. 11. Optical device 200 includes a firstoptical member 60 and a second optical member 21. Second optical member21 includes a first transparent electrode 203 formed on annularprojections 602, a TN (Twisted Nematic) type liquid crystal 204 formedthereon, a second transparent electrode 205 formed further thereon, anda transmittive substrate 206. First optical member 60 is identical tothat described previously.

TN type liquid crystal 204 is sealed between first and secondtransparent electrodes 203 and 205 so that the molecular arrangement isnot twisted 90 degrees. Therefore, the plane of polarization of thelaser beam passing through TN type liquid crystal 204 will not berotated 90 degrees.

The optical property of optical device 200 will be described withreference to FIGS. 12 and 13. Annular projections 203 forming a hologramhave a refractive index of 1.7 with respect to a laser beam of 635 nm inwavelength. TN liquid crystal 204 has a refractive index of 1.5 whenvoltage is not applied across first and second transparent electrodes203 and 205, and has a refractive index of 1.7 when voltage is applied.

Referring to FIG. 12, the laser beam of 635 nm in wavelength directlypasses through optical device 200 when voltage is applied to TN liquidcrystal 204 since projections 602 forming the hologram and TN liquidcrystal 204 have the same refractive index 1.7.

Referring to FIG. 13, the laser beam of 635 nm in wavelength isdiffracted in a desired direction to be transmitted from optical device200 when voltage is not applied to liquid crystal 204 since projections602 forming the hologram and TN type liquid crystal 204 have therefractive index of 1.7 and 1.5, respectively, and projection 602 has agentle slope.

Optical device 200 diffracts the laser beam in a desired direction whilemaintaining the incident intensity by selectively applying voltage to TNliquid crystal 204 irrespective of the wavelength of the laser beam.

Referring to FIG. 14, reproduction of DVD 11 having a substratethickness of 0.6 mm will be described. In reproducing a signal from DVD11, voltage is applied to first and second transparent electrodes 203and 205 of optical device 200. As a result, the laser beam of 635 nm inwavelength emitted from laser light source 100 is rendered parallel bycollimator lens 2 and passes through polarization beam splitter 3. Thelaser beam has its plane of polarization rotated 90 degrees by half-waveplate 4 and enters rising mirror 5. There is little reduction in thepower of the laser beam by the passage of polarization beam splitter 3and half-wave plate 4 since the laser beam is transmitted throughpolarization beam splitter 3 and half-wave plate 4 at the transmittanceof approximately 98%.

The laser beam incident to rising mirror 5 is reflected almost 100% toenter optical device 200. The laser beam incident to optical device 200is directly transmitted maintaining the incident intensity to enterobjective lens 7. The laser beam in objective lens 7 is focused to beprojected on signal recording plane 11 a of DVD 11. The light reflectedfrom signal recording plane 11 a passes through objective lens 7,optical device 200 and rising mirror 5 to return to half-wave plate 4.The light has its plane of polarization rotated 90 degrees at half-waveplate 4 and then enters polarization beam splitter 3. The reflectedlight entering polarization beam splitter 3 is reflected almost 100%thereat to enter collective lens 8 since the plane of polarization isrotated 180 degrees than the case where the light beam enterspolarization beam splitter 3 from collimator lens 2. Then, the light isfocused at collective lens 8 to be directed to photodetector 9 fordetection.

Reproduction from CD 110 having a substrate thickness of 1.2 mm will bedescribed with reference to FIG. 15. In reproducing a signal from CD110, voltage is not applied to first and second transparent electrodes203 and 205 of optical device 200. As a result, the laser beam of 635 nmin wavelength emitted from laser light source 100 is rendered parallelby collimator lens 2 and passes through polarization beam splitter 3.The light has its plane of polarization rotated 90 degrees at half-waveplate 4 to enter rising mirror 5. The laser beam is hardly reduced inpower by passage through polarization beam splitter 3 and half-waveplate 4 since the laser beam is transmitted through polarization beamsplitter 3 and half-wave plate 4 at the transmittance of approximately98%.

The laser beam incident to rising mirror is reflected almost 100% andenters optical device 200. The laser beam incident to optical device 200is diffracted in a desired direction while maintaining the incidentintensity. Only the predetermined center portion of the laser beamenters objective lens 7. The incident laser beam to objective lens 7 iscollected at objective lens 7 to be projected on signal recording plane110 a of CD 110. In this case, the diameter of the predetermined centerportion is determined so that the effective numerical aperture ofobjective lens 7 is in the range of 0.3 to 0.4. As a result, the laserbeam of 635 nm in wavelength is projected onto signal recording plane110 a of CD 110 having a substrate thickness of 1.2 mm with almost noaberration. The light reflected from signal recording plane 110 a isdetected by photodetector 9 in a manner similar to that described above.

Referring to FIG. 16, an optical pickup device 30 which is animprovement of optical pickup device 10 of FIG. 1 will be described.Optical pickup device 30 includes a rising mirror 50 instead of risingmirror 5 of optical pickup device 10. The remaining structure is similarto that of optical pickup device 10. The details of rising mirror 50disclosed in Japanese Patent Application No. 10-257130 will be describedbriefly hereinafter.

Referring to FIG. 17, rising mirror 50 includes a thin film 501 thatsets the optical axes of two laser beams LB1 and LB2 in coincidence atits surface. Since laser light source 1 has a first semiconductor laser1A and a second semiconductor laser 1B, the optical axes of laser beamsLB1 and LB2 emitted from the two semiconductor lasers will be deviatedfrom each other. Therefore, it is necessary to set the optical axes ofthe two laser beams LB1 and LB2 in coincidence in order to carry outrecording and reproduction of a signal correctly.

Optical pickup device 30 uses rising mirror 50 including a thin film 50to set the optical axis of laser beam LB1 of 635 nm in wavelength andthe optical axis of laser beam LB2 of 780 nm in wavelength incoincidence.

Laser beam LB1 of 635 nm in wavelength is reflected at a first plane5011 of thin film 501 of rising mirror 50. Laser beam LB2 of wavelength780 nm is refracted at first plane 5011 of thin film 501 of risingmirror 50 and reflected at a second plane 5012 to be refracted again atfirst plane 5011 to be output from rising mirror 50 as a laser beamhaving an optical axis identical to that of the reflected light of thelaser beam of 635 nm in wavelength.

The passage of rising mirror 50 allows the optical axes of the laserbeams LB1 and LB2 of the two wavelengths to match each other withoutreduction in the intensity of the laser beam. Therefore, a signal can berecorded and/or reproduced more accurately.

Optical devices 6, 80 and 200 described above are located at anarbitrary position between laser light source 1 and objective lens 7.

The above-described optical pickup devices 10, 20 and 30 of the presentinvention diverts the circumferential portion of the laser beam outsideobjective lens 7 in the recording and reproducing operation with respectto CD-Rn 110 using optical devices 6, 80 and 200. This means that thereis a relatively large loss. In order to output a laser beam havingsufficient power from objective lens 7, the output power of laser lightsources 1 and 100 or the numerical aperture of collimator lens 2 andobjective lens 7 should be increased. However, increase thereof has alimit. A greater power is required for the laser beam output fromobjective lens 7 as the data reading or writing speed increases. Also,it is to be noted that the effective region of collimator lens 2 andobjective lens 7 is a true circle whereas the laser beam output fromlaser light source 1 has a cross section of an ellipse, not a truecircle. If the entire laser beam in the direction of the longer diameteris made to be incident on the effective region of collimator lens 2 orobjective lens 7, there will be an effective region that is not used incollimator lens 2 and objective lens 7 in the direction of the shorterdiameter. If the laser beam in the direction of the shorter diameter ismade to be incident on the entire effective region of collimator lens 2or objective lens 7, the laser beam in the direction of the longerdiameter will be wasted partially. In general, if the design of thefocal length of collimator lens 2 is made short so that laser lightsource 1 or 100 is located close to collimator lens 2, the power of thelaser beam output from objective lens 7 will become greater. However,the spot diameter of the laser beam formed on optical disk 11 (or 110)will become too large. Therefore, the rim intensity defined below mustbe set appropriately in order to obtain sufficient output power whilemeeting various recording or reproduction conditions.

As shown in FIG. 18, the laser beam output from the laser light sourceis increased in diameter to enter the collimator lens. The laser beam isrendered parallel by the collimator lens, and then enters the objectivelens. Here, the broadening angle θ// in the direction of the shorterdiameter is smaller than the broadening angle θ⊥ in the direction of thelonger diameter.

The intensity of the laser beam corresponds to a Gaussian distributionas shown in FIG. 19. The intensity is highest at the center and becomeslower towards the outer circumference. Since the laser intensity forms aGaussian distribution, a laser beam of at least a predeterminedintensity will be used in practical usage. If the maximum laser power is100%, the rim intensity I % is defined when laser of at least I % isused.

Since the laser beam has a cross section of an ellipse, the intensitydistribution is abrupt in the shorter diameter direction (X-X′) as shownin FIG. 20 and gentle in the longer diameter direction (Y-Y′) as shownin FIG. 21. Since the effective region of the objective lens correspondsto a true circle, the rim intensity becomes smaller in the shorterdiameter direction as shown in FIG. 20 and higher in the longer diameterdirection as shown in FIG. 21.

The simulation result of the output power when the kinoform type opticaldevice 6 of the present invention shown in FIG. 3A is employed will bedescribed in comparison with the output power when the conventionalstepped optical device shown in FIG. 4C is employed.

First, an example of the rim intensity when priority is given onefficiency will be described. As shown in FIG. 22, a laser light sourcehaving a broadening angle of 7.5° in the shorter diameter direction anda broadening angle of 17° in the longer diameter direction is used. Acollimator lens having a numerical aperture NA of 0.15 with a focallength f of 9 mm is employed. The case is considered where the rimintensity is set to 0.6% in the shorter diameter direction as shown inFIG. 23 and to 36.6% in the longer diameter direction as shown in FIG.24.

By setting the effective numerical aperture NA of the objective lens to0.5, the output power of the laser light source to 70 mW and the otherparameters to appropriate values as shown in Table 1 below, the outputpower from the objective lens becomes 49.90 mW when the kinoform typeoptical device 6 is employed.

TABLE 1 Priority on Efficiency (New HOE) Pickup Design CalculationCondition Calculated value 1. Objective lens NA 0.5 f = 3.2Transmittance Objective lens 3.20 mm 95% effective diameter CollimatorAt least lens NA 0.18 2. Collimator NA 0.15 f = 9 mm TransmittanceOptical 2.81 times lens 95% Magnification Lens bond 20.48 deg angle 3.Laser Wavelength CW 70 mW Pulse 0 mW Lens bond 82.24% 780 nm efficiencyθ// 7.5 deg θ ⊥ 17 deg effective 7.50 deg θ//angle effective θ ⊥ 17.00deg angle 4. Beam θ// 1 time θ ⊥ 1 time Transmittance formation 100%magnification 5. Beam splitter Tp 100% Transmittance Pick efficiency71.28% 98% CW output 49.90 mW power 6. HOE Spectral Transmittance Pulseoutput 0.00 mW ratio 100% 100% power Rim intensity 0.57% θ// 7. Risingmirror Transmittance Rim intensity 36.57% 98% θ ⊥ θ//direction θ ⊥direction Rim intensity 0.6% 36.6% Eclipse 1.60 0.71 coefficientExpected spot 1.460 μm 1.384 μm diameter

If the effective numerical aperture NA of the objective lens isincreased to 0.53 as shown in Table 2, the output power is boosted to51.65 mW.

TABLE 2 Priority on Efficiency (New HOE) Pickup Design CalculationCondition Calculated value 1. Objective lens NA 0.53 f = 3.2Transmittance Objective lens 3.39 mm 95% effective diameter CollimatorAt least NA 0.19 2. Collimator NA 0.15 f = 9 mm Transmittance Optical2.81 times lens 95% Magnification Lens bond 21.72 deg angle 3. LaserWavelength CW 70 mW Pulse 0 mW Lens bond 85.13% 780 nm efficiency θ//7.5 deg θ ⊥ 17 deg effective 7.50 deg θ//angle effective θ ⊥ 17.00 degangle 4. Beam θ// 1 time θ ⊥ 1 time Transmittance formation 100%magnification 5. Beam splitter Tp 100% Transmittance Pick efficiency73.79% 98% CW output 51.65 mW power 6. HOE Spectral Transmittance Pulseoutput 0.00 mW ratio 100% 100% power Rim intensity 0.30% θ// 7. Risingmirror Transmittance Rim intensity 32.24% 98% θ ⊥ θ//direction θ ⊥direction Rim intensity 0.3% 32.2% Eclipse 1.70 0.76 coefficientExpected spot 1.378 μm 1.314 μm diameter

When the conventional stepped optical device is employed underconditions identical to those of Table 1 as in Table 3 below, the outputpower is degraded to 39.92 mW.

TABLE 3 Priority on Efficiency (Conventional HOE) Pickup DesignCalculation Condition Calculated value 1. Objective lens NA 0.5 f = 3.2Transmittance Objective lens 3.20 mm 95% effective diameter CollimatorAt least lens NA 0.18 2. Collimator NA 0.15 f = 9 mm TransmittanceOptical 2.81 times lens 95% Magnification Lens bond 20.48 deg angle 3.Laser Wavelength CW 70 mW Pulse 0 mW Lens bond 82.24% 780 nm efficiencyθ// 7.5 deg θ ⊥ 17 deg effective 7.50 deg θ//angle effective θ ⊥ 17.00deg angle 4. Beam θ// 1 time θ ⊥ 1 time Transmittance formation 100%magnification 5. Beam splitter Tp 100% Transmittance Pick efficiency57.03% 98% CW output 39.92 mW power 6. HOE Spectral Transmittance Pulseoutput 0.00 mW ratio 80% 100% power Rim intensity 0.57% θ// 7. Risingmirror Transmittance Rim intensity 36.57% 98% θ ⊥ θ//direction θ ⊥direction Rim intensity 0.6% 36.6% Eclipse 1.60 0.71 coefficientExpected spot 1.460 μm 1.384 μm diameter

When the conventional stepped optical device is employed under theconditions identical to those of Table 2 as in Table 4 below, the outputpower is degraded to 41.32 mW.

TABLE 4 Priority on Efficiency (Conventional HOE) Pickup DesignCalculation Condition Calculated value 1. Objective lens NA 0.53 f = 3.2Transmittance Objective lens 3.39 mm 95% effective diameter CollimatorAt least lens NA 0.19 2. Collimator NA 0.15 f = 9 mm TransmittanceOptical 2.81 times lens 95% Magnification Lens bond 21.72 deg angle 3.Laser Wavelength CW 70 mW Pulse 0 mW Lens bond 85.13% 780 nm efficiencyθ// 7.5 deg θ ⊥ 17 deg effective 7.50 deg θ//angle effective θ ⊥ 17.00deg angle 4. Beam θ// 1 time θ ⊥ 1 time Transmittance formation 100%magnification 5. Beam splitter Tp 100% Transmittance Pick efficiency59.03% 98% CW output 41.32 mW power 6. HOE Spectral Transmittance Pulseoutput 0.00 mW ratio 80% 100% power Rim intensity 0.30% θ// 7. Risingmirror Transmittance Rim intensity 32.24% 98% θ ⊥ θ//direction θ ⊥direction Rim intensity 0.3% 32.2% Eclipse 1.70 0.76 coefficientExpected spot 1.378 μm 1.314 μm diameter

The rim intensity when priority is given on the spot size will bedescribed here. As shown in FIG. 25, a laser light source having abroadening angle of 7.5° in the shorter diameter direction and 17° inthe longer diameter direction is employed. A collimator lens having aneffective numerical aperture NA of 0.15 at the focal length f of 20 mmis employed. The case is considered where the rim intensity is set to35.4% in the shorter diameter direction as shown in FIG. 26 and to 81.7%in the longer diameter direction as shown in FIG. 27.

By setting the effective numerical aperture NA of the objective lens to0.5, the output power of the laser light source to 70 mW and the otherparameters to appropriate values as shown in Table 5 below, the outputpower of the objective lens becomes 20.41 mW when using the kinoformtype optical device 6. Although the output power becomes lower than thatcorresponding to Table 1 where priority is given on the efficiency, thespot size becomes smaller than that of Table 1.

TABLE 5 Priority on spot size (New HOE) Pickup Design CalculationCondition Calculated value 1. Objective lens NA 0.5 f = 3.2Transmittance Objective lens 3.20 mm 95% effective diameter CollimatorAt least lens NA 0.08 2. Collimator NA 0.15 f = 20 mm TransmittanceOptical 6.25 times lens 95% Magnification Lens bond 9.18 deg angle 3.Laser Wavelength CW 70 mW Pulse 0 mW Lens bond 33.64% 780 nm efficiencyθ// 7.5 deg θ ⊥ 17 deg effective 7.50 deg θ//angle effective θ ⊥ 17.00deg angle 4. Beam θ// 1 time θ ⊥ 1 time Transmittance formation 100%magnification 5. Beam splitter Tp 100% Transmittance Pick efficiency29.15% 98% CW output 20.41 mW power 6. HOE Spectral Transmittance Pulseoutput 0.00 mW ratio 100% 100% power Rim intensity 35.42% θ// 7. Risingmirror Transmittance Rim intensity 81.71% 98% θ ⊥ θ//direction θ ⊥direction Rim intensity 35.4% 81.7% Eclipse 0.72 0.32 coefficientExpected spot 1.386 μm 1.313 μm diameter

If the effective numerical aperture NA of the objective lens isincreased to 0.53 of the objective lens as shown in Table 6, the outputpower increases to 22.05 mW. Although this output power becomes lowerthan that of Table 2 where priority is given on the efficiency, the spotsize becomes smaller than that of Table 2.

TABLE 6 Priority on spot size (New HOE) Pickup Design CalculationCondition Calculated value 1. Objective lens NA 0.53 f = 3.2Transmittance Objective lens 3.39 mm 95% effective diameter CollimatorAt least lens NA 0.08 2. Collimator NA 0.15 f = 20 mm TransmittanceOptical 6.25 times lens 95% Magnification Lens bond 9.73 deg angle 3.Laser Wavelength CW 70 mW Pulse 0 mW Lens bond 36.37% 780 nm efficiencyθ// 7.5 deg θ ⊥ 17 deg effective 7.50 deg θ//angle effective θ ⊥ 17.00deg angle 4. Beam θ// 1 time θ ⊥ 1 time Transmittance formation 100%magnification 5. Beam splitter Tp 100% Transmittance Pick efficiency31.52% 98% CW output 22.06 mW power 6. HOE Spectral Transmittance Pulseoutput 0.00 mW ratio 100% 100% power Rim intensity 31.15% θ// 7. Risingmirror Transmittance Rim intensity 79.69% 98% θ ⊥ θ//direction θ ⊥direction Rim intensity 31.1% 79.7% Eclipse 0.76 0.34 coefficientExpected spot 1.316 μm 1.241 μm diameter

When a conventional stepped optical device is employed under conditionsidentical to those of the above Table 5 as in Table 7 below, the outputpower is reduced to 16.33 mW.

TABLE 7 Priority on spot size (Conventional HOE) Pickup DesignCalculation Condition Calculated value 1. Objective lens NA 0.5 f = 3.2Transmittance Objective lens 3.20 mm 95% effective diameter CollimatorAt least lens NA 0.08 2. Collimator NA 0.15 f = 20 mm TransmittanceOptical 6.25 times lens 95% Magnification Lens bond 9.18 deg angle 3.Laser Wavelength CW 70 mW Pulse 0 mW Lens bond 33.64% 780 nm efficiencyθ// 7.5 deg θ ⊥ 17 deg effective 7.50 deg θ//angle effective θ ⊥ 17.00deg angle 4. Beam θ// 1 time θ ⊥ 1 time Transmittance formation 100%magnification 5. Beam splitter Tp 100% Transmittance Pick efficiency23.32% 98% CW output 16.33 mW power 6. HOE Spectral Transmittance Pulseoutput 0.00 mW ratio 80% 100% power Rim intensity 35.42% θ// 7. Risingmirror Transmittance Rim intensity 81.71% 98% θ ⊥ θ//direction θ ⊥direction Rim intensity 35.4% 81.7% Eclipse 0.72 0.32 coefficientExpected spot 1.386 μm 1.313 μm diameter

When a conventional stepped optical device is employed under conditionsidentical to those of the above Table 6 as in Table 8 below, the outputpower is reduced to 17.65 mW.

TABLE 8 Priority on spot size (Conventional HOE) Pickup DesignCalculation Condition Calculated value 1. Objective lens NA 0.53 f = 3.2Transmittance Objective lens 3.39 mm 95% effective diameter CollimatorAt least lens NA 0.08 2. Collimator NA 0.15 f = 20 mm TransmittanceOptical 6.25 times lens 95% Magnification Lens bond 9.73 deg angle 3.Laser Wavelength CW 70 mW Pulse 0 mW Lens bond 36.37% 780 nm efficiencyθ// 7.5 deg θ ⊥ 17 deg effective 7.50 deg θ//angle effective θ ⊥ 17.00deg angle 4. Beam θ// 1 time θ ⊥ 1 time Transmittance formation 100%magnification 5. Beam splitter Tp 100% Transmittance Pick efficiency25.22% 98% CW output 17.65 mW power 6. HOE Spectral Transmittance Pulseoutput 0.00 mW ratio 80% 100% power Rim intensity 31.15% θ// 7. Risingmirror Transmittance Rim intensity 79.69% 98% θ ⊥ θ//direction θ ⊥direction Rim intensity 31.1% 79.7% Eclipse 0.76 0.34 coefficientExpected spot 1.316 μm 1.241 μm diameter

Next, an example of the rim intensity suitable for an eightfold-speedoptical pickup device will be described. As shown in FIG. 28, a laserlight source having a broadening angle of 7.5° in the shorter diameterdirection and 17° in the longer diameter direction is employed. Acollimator lens having an effective numerical aperture NA of 0.15 with afocal length f of 15 mm is employed. The case is considered where therim intensity is set to 15.8% in the shorter diameter direction as shownin FIG. 29 and to 69.8% in the longer diameter direction as shown inFIG. 30.

When the effective numerical aperture NA of the objective lens is set to0.5, the output power of the laser beam set to 70 mW, and the otherparameters set to appropriate values as shown in Table 9, the outputpower from the objective lens becomes 30.62 mW when using the kinoformtype optical device 6. Since recording and reproduction at the eightfoldspeed is possible if the output power is at least 30 mW, operationthereof is possible in this case.

TABLE 9 Eightfold-speed pickup (New HOE) Pickup Design CalculationCondition Calculated value 1. Objective lens NA 0.5 F = 3.2Transmittance Objective lens 3.20 mm 95% effective diameter CollimatorAt least lens NA 0.11 2. Collimator NA 0.15 f = 15 mm TransmittanceOptical 4.69 times lens 95% Magnification Lens bond 12.25 deg angle 3.Laser Wavelength CW 70 mW Pulse 0 mW Lens bond 50.47% 780 nm efficiencyθ// 7.5 deg θ ⊥ 17 deg effective 7.50 deg θ//angle effective θ ⊥ 17.00deg angle 4. Beam θ// 1 time θ ⊥ 1 time Transmittance formation 100%magnification 5. Beam splitter Tp 100% Transmittance Pick efficiency43.75% 98% CW output 30.62 mW power 6. HOE Spectral Transmittance Pulseoutput 0.00 mW ratio 100% 100% power Rim intensity 15.75% θ// 7. Risingmirror Transmittance Rim intensity 69.79% 98% θ ⊥ θ//direction θ ⊥direction Rim intensity 15.8% 69.8% Eclipse 0.96 0.43 coefficientExpected spot 1.428 μm 1.326 μm diameter

When the effective numerical aperture NA of the objective lens isincreased to 0.53 as shown in Table 10, the output power from theobjective lens becomes 33.32 mW. Recording and reproduction at theeightfold speed is possible in this case.

TABLE 10 Eightfold-speed pickup (New HOE) Pickup Design CalculationCondition Calculated value 1. Objective lens NA 0.53 F = 3.2Transmittance Objective lens 3.39 mm 95% effective diameter CollimatorAt least lens NA 0.11 2. Collimator NA 0.15 f = 15 mm TransmittanceOptical 4.69 times lens 95% Magnification Lens bond 12.98 deg angle 3.Laser Wavelength CW 70 mW Pulse 0 mW Lens bond 54.92% 780 nm efficiencyθ// 7.5 deg θ ⊥ 17 deg effective 7.50 deg θ//angle effective θ ⊥ 17.00deg angle 4. Beam θ// 1 time θ ⊥ 1 time Transmittance formation 100%magnification 5. Beam splitter Tp 100% Transmittance Pick efficiency47.60% 98% CW output 33.32 mW power 6. HOE Spectral Transmittance Pulseoutput 0.00 mW ratio 100% 100% power Rim intensity 12.52% θ// 7. Risingmirror Transmittance Rim intensity 66.74% 98% θ ⊥ θ//direction θ ⊥direction Rim intensity 12.5% 66.7% Eclipse 1.02 0.45 coefficientExpected spot 1.354 μm 1.255 μm diameter

When a conventional stepped optical device is employed under conditionsidentical to those of Table 9 as in Table 11 below, the output powerfrom the objective lens is reduced to 24.50 mW. In this case, recordingand reproduction at eightfold speed is not possible.

TABLE 11 Eightfold-speed pickup (Conventional HOE) Pickup DesignCalculation Condition Calculated value 1. Objective lens NA 0.5 f = 3.2Transmittance Objective lens 3.20 mm 95% effective diameter CollimatorAt least lens NA 0.11 2. Collimator NA 0.15 f = 15 mm TransmittanceOptical 4.69 times lens 95% Magnification Lens bond 12.25 deg angle 3.Laser Wavelength CW 70 mW Pulse 0 mW Lens bond 50.47% 780 nm efficiencyθ// 7.5 deg θ ⊥ 17 deg effective 7.50 deg θ//angle effective θ ⊥ 17.00deg angle 4. Beam θ// 1 time θ ⊥ 1 time Transmittance formation 100%magnification 5. Beam splitter Tp 100% Transmittance Pick efficiency35.00% 98% CW output 24.50 mW power 6. HOE Spectral Transmittance Pulseoutput 0.00 mW ratio 80% 100% power Rim intensity 15.75% θ// 7. Risingmirror Transmittance Rim intensity 69.79% 98% θ ⊥ θ//direction θ ⊥direction Rim intensity 15.8% 69.8% Eclipse 0.96 0.43 coefficientExpected spot 1.428 μm 1.326 μm diameter

When a conventional stepped optical device is employed under conditionsidentical to those of Table 10 as in Table 12 below, the output powerfrom the objective lens is reduced to 26.66 mW. In this case, recordingand reproduction at eightfold speed is not possible.

TABLE 12 Eightfold-speed pickup (Conventional HOE) Pickup DesignCalculation Condition Calculated value 1. Objective lens NA 0.53 f = 3.2Transmittance Objective lens 3.39 mm 95% effective diameter CollimatorAt least lens NA 0.11 2. Collimator NA 0.15 f = 15 mm TransmittanceOptical 4.69 times lens 95% Magnification Lens bond 12.98 deg angle 3.Laser Wavelength CW 70 mW Pulse 0 mW Lens bond 54.92% 780 nm efficiencyθ// 7.5 deg θ ⊥ 17 deg effective 7.50 deg θ//angle effective θ ⊥ 17.00deg angle 4. Beam θ// 1 time θ ⊥ 1 time Transmittance formation 100%magnification 5. Beam splitter Tp 100% Transmittance Pick efficiency38.08% 98% CW output 26.66 mW power 6. HOE Spectral Transmittance Pulseoutput 0.00 mW ratio 80% 100% power Rim intensity 12.52% θ// 7. Risingmirror Transmittance Rim intensity 66.74% 98% θ ⊥ θ//direction θ ⊥direction Rim intensity 12.5% 66.7% Eclipse 1.02 0.45 coefficientExpected spot 1.354 μm 1.255 μm diameter

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. An optical pickup apparatus recording and/orreproducing a signal onto/from a first optical disk and a second opticaldisk thicker than said first optical disk, comprising: a light sourcegenerating a laser beam; an objective lens located opposite to saidfirst or second optical disks; and an optical device arranged betweensaid light source and said objective lens to directly transmit the laserbeam from said light source while maintaining a laser beam intensityduring recording or reproduction of said first optical disk, and bendingsubstantially the laser beam from said light source substantiallyentirely to increase the diameter of the laser beam to guide the centerportion of said laser beam to said objective lens and guiding theperipheral portion of said laser beam outside said objective lens whilemaintaining substantially a laser beam intensity during recording orreproduction of said second optical disk.
 2. The optical pickup deviceaccording to claim 1, wherein said optical device comprises a firstoptical member (60, 810) having a first refractive index, and a secondoptical member (61, 801, 21) in contact with said first optical member,and having a first refractive index upon recording or reproduction ofsaid first optical disk, and a second refractive index differing fromsaid first refractive index upon recording or reproduction of saidsecond optical disk.
 3. The optical pickup device according to claim 2,wherein said light source generates a first laser beam having a firstwavelength during recording or reproduction of said first optical disk,and generates a second laser beam having a second wavelength differingfrom said first wavelength upon recording or reproduction of said secondoptical disk, said first optical member having said first refractiveindex at said first and second wavelengths, and said second opticalmember having said first refractive index at said first wavelength andsaid second refractive index at said second wavelength.
 4. The opticalpickup device according to claim 3, wherein said first optical member(60) includes a hologram (601) formed to be in contact with said secondoptical member.
 5. The optical pickup device according to claim 4,wherein said first optical member is arranged at said light source side,said second optical member is arranged at said objective lens side, saidfirst refractive index is higher than said second refractive index. 6.The optical pickup device according to claim 4, wherein said hologramincludes a plurality of annular projections (602) formed concentrically.7. The optical pickup device according to claim 6, wherein said annularprojections have a pitch smaller as towards the circumference.
 8. Theoptical pickup device according to claim 6, wherein each of said annularprojections has a triangular cross section radially.
 9. The opticalpickup device according to claim 3, wherein said first optical member(810) is arranged at said objective lens side, and has a concave curveplane (802) in contact with said second optical member (801), saidsecond optical member (801) is arranged at said light source side, andhas a convex curve plane (802) in contact with said first optical member(810), said first refractive index is higher than said second refractiveindex.
 10. The optical pickup device according to claim 3, wherein saidlight source includes a first semiconductor laser (1A) generating saidfirst laser beam, a second semiconductor laser (1B) generating saidsecond laser beam, said optical pickup device further comprising anoptical axis correction device (501) setting an optical axis of saidfirst laser beam and an optical axis of said second laser beam incoincidence.
 11. The optical pickup device according to claim 2, whereinsaid second optical member (21) includes first and second transparentelectrodes (203, 205) opposite to each other, and liquid crystal (204)sandwiched between said first and second transparent electrodes.
 12. Theoptical pickup device according to claim 11, wherein said first opticalmember (60) includes a hologram (601) formed to be in contact with saidsecond optical member.
 13. The optical pickup device according to claim12, wherein said first optical member is arranged at said light sourceside, said second optical member is arranged at said objective lensside, said first refractive index is higher than said second refractiveindex.
 14. The optical pickup device according to claim 12, wherein saidhologram includes a plurality of annular projections (602) formedconcentrically.
 15. The optical pickup device according to claim 14,wherein said annular projections have a pitch smaller as towards thecircumference.
 16. The optical pickup device according to claim 14,wherein each of said annular projections has a triangular cross sectionradially.
 17. The optical pickup device according to claim 1, furthercomprising a collimator lens (2) having a focus where said opticalsource (1, 100) is positioned, wherein a shorter diameter of a laserbeam collimated by said collimator lens (2) is longer than an effectivediameter of said objective lens (7).